The invention relates to a high-pressure metal halide discharge lamp comprising a light-transmitting lamp vessel which is sealed in a vacuumtight manner and contains an ionizable filling including an inert gas and a metal halide, tungsten electrodes are provided with an emitter comprising, as a main constituent, tungsten which includes at least a first oxide selected from hafnium oxide and zirconium oxide, and furthermore at least a further oxide selected from the group of oxides of the elements yttrium, lanthane and the lanthanides, the electrodes and emitter being substantially free of thorium oxide.
Such a high-pressure metal halide discharge lamp is known from EP-A1-0 647 964 U.S. Pat. No. 5,30,317). This lamp has an emission spectrum and a color point which are determined, inter alia, by its ionizable filling. And, at or near their free ends, the electrodes of the known lamp are provided with an emitter. The emitter comprises 7 to 30% by volume of oxides.
An electrode having an emitter has a lower electron work function and, as a result, its temperature during operation is lower than that of an electrode without an emitter. Consequently, evaporation of electrode (and emitter) material and the deposition thereof on the lamp vessel occur to a smaller degree. As a result, the lamp having an electrode with an emitter has a better maintenance; its luminous efficacy (lm/W) during its service life exhibits a smaller decrease than that of a lamp having an electrode without an emitter. A second property of an emitter is that it leads to a shorter glow time during starting of the lamp. As a result, the starting behavior of the lamp is better and less electrode (and emitter) material is sputtered onto the wall, resulting in a better maintenance.
It is also known that a combination of the oxides in tungsten yields an emitter having suitable properties in a high-pressure metal halide discharge lamp. Although the electrodes of the known high-pressure metal halide discharge lamp are substantially free of thorium oxide, the electrodes have an at least substantially equal work function potential. This is remarkable because essential properties which are combined in thorium oxide are not present in the individual oxides used in the emitter. Consequently, in the first instance, the conclusion should be drawn that the individual oxides are hardly suitable for use as an emitter. A possible explanation for the positive effect of the combination of oxides could be that the first oxide has formed a compound, for example having a fluorite structure, with the further oxide.
The tungsten of the emitter has a grain structure with grain boundaries. In this structure, the fluorites demonstrate a great stability and immobility. The fluorites are so immobile that they hardly, or not at all, diffuse from the mass of the emitter along the grain boundaries to an electron-emitting surface. As a result, the supply of oxides present in this form in the emitter to the electron-emitting surface is reduced very substantially. It has been found however, that, in the known lamp, the discontinuation of the supply of oxide leads, during the service life, to a premature depletion of the electron-emitting surface. This premature depletion is counteracted in the known lamp by using emitters having relatively large quantities of oxides, for example the first oxide.
The known lamp comprising electrodes with the known emitter has the disadvantage that much oxide evaporates in the early stage of the service life of the lamp, which can be attributed to the presence of the oxides in relatively high concentrations in the electron-emitting surface. This results, on the one hand, in the maintenance again lagging behind that of lamps with thorium, because the relatively rapidly evaporated oxide deposits on the lamp vessel and hence adversely affects the light transmissivity, and, on the other hand, in a relatively large change in color point as a result of a reaction of the oxides with the ionizable filling present in the lamp. As a result of this reaction, during the service life, a change of the gas composition of the filling of the lamp takes place when the lamp is in the operational state. The color point with co-ordinates (x, y, z) changes particularly in its x-co-ordinate.